Signaling and gene activation mechanisms in the initial phase of effector CD4 T cell responses restrict the expression of cytokine and trafficking genes to functionally distinct programs. Production of IFN-? by Th1 cells is central to immunity against many intracellular pathogens, but induction of this gene is blocked in activated Th2 cells. In contrast to the information available about mechanisms programming the acute effector response, relatively little is known about molecular regulation in post-effector populations. Antigenic experience is central to the diverse processes involved in immunologic memory. Memory is essential for the efficacy of vaccines, the most cost-effective of medical interventions and a key to potential protection against bioweapons, so a better understanding of the molecular regulation of antigen (Ag)-experienced helper cells is a vital priority. The goal of this research program is to elucidate molecular mechanisms involved in programming cytokine gene activity in CD4 T cells that have had prior antigenic experience and then become quiescent after their first period of activation. The central hypothesis of this proposal is that commitment to one aspect of Th2 identity - chromatin marks establishing transcriptional competence of the Th2 cytokine locus - is maintained in Ag-experienced cells by persistence of GATA3 protein after initial development, but this committed population regains a capacity to activate T-bet, the IFN-? locus, and perhaps other Th1-specific genes. Using CD4 T cells purified via a lineage-marking approach, we have evidence that Ag-experienced Th2 cells can become IFN-? producers with a substantial efficiency once they have been parked in vivo for weeks, yet the population and the individual IFN-3-producing cells retain a commitment to reactivation of their Th2 program. The proposed studies will answer a few main questions: (a) What mechanisms give rise to the flexibility in IFN-? gene activation, which should be 'forbidden'in a "differentiated Th2 cell", and what are the consequences? (b) In T cells that differentiated from cells with an active IL-4 locus, is commitment to transcriptional competence of the IL-4 gene fixed or can its molecular programming be altered if expression of a developmental regulator is terminated? To answer these questions, we will analyze Th1-like flexibility (Ifng locus &other endpoints) and Th2 locus commitment after Th1-biasing immune challenges in vivo, e.g. with influenza virus (Specific Aim 1). Further, we will elucidate mechanisms by which IFN-? gene expression is induced in memory-like Th2 cells after their re-activation and growth in Th1 conditions (Aim 2). In Aim 3, we will determine if persistence of GATA3, a master regulator of initial Th2 locus activation, is required for maintenance of IL-4 locus competence and chromatin marks in the population derived from Ag-experienced IL-4 locus-activated CD4 T cells. We propose that loss of the transcription factor GATA-3 from differentiated Th2 cells will alter the functional capabilities of their recall response in terms of localization, IL-4 production, &allergic inflammation. Together, the proposed studies will provide new insights into the programming of antigen-experienced helper populations. PUBLIC HEALTH RELEVANCE: The properties of T lymphocytes that have previously been activated by the target for which they are specific are crucial for normal immunity to pathogens, effective vaccines, and biodefense. This type of "antigen-experienced" cell contributes in important ways to immune memory, and also influences flares of allergic disease such as asthma. By exploring a new and unexpected finding about flexibility in the gene expression program of these cells, the proposed research will generate important new insights into the regulation of which genes get expressed in helper T cells, the quarterbacks of adaptive immunity, after cells have been activated once by antigen, picked up specific types of function, and then gone into a resting state.